1. Field of the Invention
The present invention generally relates to the field of semiconductor fabrication. More particularly, the invention relates to the fabrication of acoustic wave structures in semiconductor dies.
2. Background Art
Bulk acoustic wave (BAW) resonators, which can be used in frequency control or filtering applications, can include a piezoelectric layer sandwiched between upper and lower electrodes and an underlying acoustic mirror. When an electric field is applied across the piezoelectric layer via the upper and lower electrodes, electrical energy is converted into acoustic energy in the piezoelectric layer through electromechanical coupling, thereby causing the piezoelectric layer to vibrate and generate acoustic waves. However, for a BAW resonator to operate effectively, the piezoelectric layer must be highly textured, i.e., it must have an oriented grain structure. To grow a highly textured piezoelectric layer, however, the grain structure of the underlying lower electrode on which the piezoelectric layer is grown must be properly oriented.
Conventional approaches for growing a highly textured piezoelectric layer typical include extensive underlying layer surface treatment, which can include chemical mechanical polishing (CMP) and/or reactive etch treatment, and a carefully chosen seed layer prior to lower electrode deposition to achieve a lower electrode on which the highly textured piezoelectric layer can be grown. However, the extensive underlying layer surface treatment typically required to achieve a highly textured piezoelectric layer in the conventional BAW resonator can undesirably increase process complexity and manufacturing cost.
Also, for increasing electromechanical coupling, which is important for BAW resonator performance, it is highly desirable to place a high density metal adjacent to the top and bottom surfaces of the piezoelectric layer. Thus, a high density metal, such as molybdenum or tungsten, is typically utilized to form the lower and the upper electrode of the conventional BAW resonator. However, although effective for increasing electromechanical coupling, a high density metal has a relatively high resistance, which can increase electrical loss in the BAW resonator and, thereby, undesirably reduce resonator performance.